A conventional technology is employed to detect misfire caused in an internal combustion engine according to a rotational state of the engine. For example, Patent Document 1 discloses a configuration to perform frequency analysis on a rotation signal, which represents an engine revolution of a four-stroke engine having four cylinders. The configuration detects misfire caused in the engine according to an analysis result of the frequency analysis. The configuration of Patent Document 1 compares a frequency component of an ignition cycle of one of the four cylinders with a frequency component of a transition cycle, at which the four cylinders sequentially moves to its top dead center state in order. The configuration of Patent Document 1 further detects occurrence of misfire in a case where the frequency component of the ignition cycle is greater than the frequency component of the transition cycle and where a difference therebetween is greater than a threshold value. When misfire does not occur in the internal combustion engine, the frequency component of the ignition cycle decreases to be smaller than the frequency component of the transition cycle.
(Patent Documents 1)
Japanese published unexamined application No. H4-19344.
It is noted that, in the configuration to detect misfire according to the magnitude of the frequency component of the ignition cycle, it is concerned that a detection accuracy would decrease in a case where noise arises in the rotation signal. For example, in a vehicle equipped with an internal combustion engine, a disturbance may arise as the vehicle travels. The disturbance is, for example, vibration caused from a road surface, an axle, and/or the like. In a case where such a disturbance is carried as a noise on the rotation signal, the cycle of the noise may coincide with the ignition cycle. When an occurrence cycle of noise coincides with the ignition cycle and when the frequency component of the ignition cycle is greater than the frequency component of the transition cycle, the configuration may detect misfire even though misfire does not occur in the internal combustion engine.